When I was growing up the rule in our house was that we could have a "friend" party on our even birthdays and "family" parties on the odd birthdays. I didn't mind this as a kid, and now as a mom I totally understand why my parents came up with this rule...parties are a LOT of work.
So as I started having children and those children starting having birthdays I threw my first "friend" party. It was for my first son's 3rd birthday...now it wasn't a disaster, but after all was said and done I thought, "it was fun, but I don't think he even cared whether his friends were there or not...and really he has friends, but mostly they are my friends kids.." Then I remembered the rule from my childhood and decided that we would adopt it into our family with their first "friend" party starting when they turn 6.
So far this has worked fantastically! By the time they turn 6 they have a fun cluster of their own friends to invite and they are old enough to really enjoy having them all over to celebrate.
I have also noticed that my kids enjoy our "family" parties too. I enjoy the more cozy nature that a family party creates, and because the friend parties are less often when they do come around it is funner to plan and do them.
Why the "friend" parties on the even birthdays?...well all the really important age milestones seem to fall on even birthdays...8...12...16...18...
Having this rule in place since my kids have been young means that they don't really even question or throw a fit about wanting a friend party...(so far)!